This invention relates generally to axial impeller, and more particularly to automatically controlled variable pitch blade impellers that may be used for example in unsteady back pressure and discharge flow regimes.
Many axial fan systems are subject to large variations in the amplitude of back pressure and therefore, fan discharge flow rate. Prior art designs generally employ fixed or manually variable fan blades or airfoils which will encounter these aforementioned large variations in back pressure resulting in large variations in apparent angle-of-attack of the blades. For a selected blade pitch the blade airfoil will stall aerodynamically with too much back pressure, and thereby cease to provide the aerodynamic lift that provides air flow. Conversely, with little or no back pressure, the impeller blade airfoils are inefficient, and drive power is wasted and over speed may occur. The overall effect of airfoil stall and other unsteady mechanisms, such as fan flow hysteresis (non-linear response), is a reduction in fan performance efficiency. In other words, it takes a while for the blade airfoils to recover from the stall to become effective again.